U.S. Pat. No. 5,220,112 by Bucci et al. teaches a thermal treatment process for the stabilization of heavy metals-containing fly ash comprising forming a mixture of the fly ash and a calcium-containing material, heating the mixture to a temperature in the range of from about 475.degree. C. to about 650.degree. C. and maintaining said temperature for a period of time from about 30 minutes to about five hours while in the presence of an oxygen-containing gas to produce a thermally treated fly ash and a purge gas.
There are several problems, however, relating to the temperature range in Bucci as follows:
(1) At temperatures below 400.degree. C., complete mercury recovery of any mercury which may be contained in the fly ash is not possible. Elemental mercury will not volatize below 357.degree. C. and mercury(I) chloride, suspected to form in reactions between mercury and halogen gases, will remain a solid below 400.degree. C. Additionally, dioxins are only partially destroyed at temperatures below 400.degree. C. PA1 (2) At temperatures above 600.degree. C., elemental arsenic and selenium will volatize and subsequently contaminate the purge stream from the thermal treatment process. PA1 (a) narrowing said temperature range from about 475.degree. C.-650.degree. C. to about 475.degree. C.-600.degree. C.; PA1 (b) separating the thermally treated fly ash from the purge gas; and PA1 (c) cooling the separated purge gas to a temperature below 357.degree. C. in order to condense out and recover at least a portion of the mercury.
It is an object of the present invention to overcome these problems by narrowing the temperature range in Bucci.